I. Field
The following description relates generally to a thermal detection circuit, and more particularly to a circuit and method of auto-ranging thermal detection.
II. Background
Thermistors are widely used for detecting temperature in typical thermal protection circuits for electronic devices. A thermistor can be a positive temperature coefficient (PTC)-type or a negative temperature coefficient (NTC)-type. The resistance of a PTC-type thermistor increases as its temperature increases; and the resistance of an NTC-type thermistor decreases as its temperature increases. In either case, the resistance of either type of thermistor can be used to determine the temperature of the body of the thermistor, which is indicative of the environmental temperature, and to warn of either an over-temperature state or an under-temperature state if the resistance of the thermistor crosses a predetermined threshold.
Unfortunately, typical systems implementing thermistors to monitor environmental temperature have particular drawbacks. For detecting the variation in resistance of a thermistor, a typical approach is to couple a constant voltage source or a constant current source as an input to the thermistor. A comparator is used to compare the voltage measured across the thermistor to a threshold voltage level. As the resistance of the thermistor rises or falls due to a change in temperature, the measured voltage will also change as the current flowing through the thermistor does not. If the voltage of the thermistor reaches the threshold voltage level, the comparator will detect this and output a corresponding signal.
Because of the static nature constant current/voltage sources, and the range of change of the resistance of the thermistor being very large, the voltage sensed at the thermistor may not be large enough at certain limits of the full range of temperatures under which the circuit is supposed to operate when compared to an offset range of the comparator. This may result in reduced accuracy of temperature detection and ultimately could result in an over- or under-heated and damaged system. Consequently, it would be desirable to address one or more of the deficiencies described above.